The background of the invention will be set forth in two parts.
1. Field of the Invention
The invention relates to surface acoustic wave devices, and more particularly to such devices having temperature stability.
2. Description of the Prior Art
In recent years there has been much interest in techniques for achieving zero temperature coefficient of delay (TCD) in surface acoustic wave structures. One such technique is to choose a crystalline cut and orientation which has zero TCD, as described in an article entitled "The Temperature Coefficient of Delay-Time for X-Propagating Acoustic Surface-Waves on Rotated Y-Cuts of .alpha. Quartz," by J. F. Dias, H. E. Karrer, J. A. Kusters, J. H. Matsinger, and M. B. Shultz, in IEEE Transactions on Sonics and Ultrasonics, SU-22, pages 46-50 (1975). Another technique was to use film overlays on the propagating surface to constrain the TCD, as described in a paper by T. E. Parker and M. B. Shultz, "SiO.sub.2 Film Overlays for Temperature-Stable Surface Acoustic Wave Devices," in Applied Physics Letters, Vol. 26, pages 75-77 (1975). The desired zero temperature coefficient of delay has also been provided by applying an external strain to a crystal, as described by M. Toda and S. Osaka in an article entitled "Temperature-Independent-Time-Delay Surface Acoustic-Wave Device Using a LiNbO.sub.3 -Bimetallic Plate Structure," in the IEEE Transactions on Sonics and Ultrasonics, Vol. SU-22, pages 39-45 (1975). Further, the use of bonded acoustic composites has been proposed by Michael T. Wauk in an article entitled "LiNbO.sub.3 -Quartz-LiNbO.sub.3 Composite Delay Line With Zero Linear Temperature Coefficient of Delay" in Electronics Letters, Vol. 10, pages 109-110 (1974). In the first three above-mentioned techniques, the temperature characteristics are fixed in the initial fabrication, while in the fourth, path length tradeoff is possible, but fabrication of the bonded composite structure is difficult.
An advantageous end use of such zero TCD surface acoustic wave structures is a temperature-stable feedback oscillator. Such oscillators using surface acoustic wave filters on ST quartz are currently receiving considerable attention. This is due in large measure to the planar, rugged and cost-effective features with which these oscillators can be built, well into the microwave frequency range. For more complete background in this area, reference may be had to an article by M. F. Lewis entitled "Some Aspects of SAW Oscillators" in 1973, IEEE Ultrasonics Symposium Proceedings, IEEE Cat. No. 73CH0807-8SU, Monterey, California, page 344. The frequency-determining component in these devices is a pair of thin-metal-film (usually Al) interdigital transducers spaced at a particular delay distance on the quartz substrate, so that the resulting narrow band filter characteristic selects only one from the many possible modes for feedback oscillation.
In conventional feedback oscillator filters the transducer patterns that produce mode selection generally require large fractional metallizations along the acoustic paths proportional to the oscillator Q. At higher Q's and at high frequencies, cumulative coherent reflections from many periodically spaced metal fingers and excess propagation loss due to large fractional metallizations become prohibitive. In addition, the temperature stable turnaround point on ST quartz (nominally 25.degree. C for a free x-propagating ST quartz surface) decreases rapidly to the negative centigrade range with increasing metal coverage and thickness. Corrective measures have been proposed to at least partially alleviate or compensate for these phenomena. For more complete data in this area, reference may be had to the above-noted Lewis article, the Dias et al article, and such articles as "Design of Harmonic Surface Acoustic Wave (SAW) Oscillators Without External Filtering and New Data on the Temperature Coefficient of Quartz" by S. J. Kerbel in the 1974 IEEE Ultrasonics Symposium Proceedings, IEEE Cat. No. 74CH 0896-ISU, Milwaukee, Wisconsin, page 276, and an article by M. B. Shultz, B. J. Matsinger, and M. G. Holland, entitled "Temperature Dependence of Surface Acoustic Wave Velocity on .alpha. Quartz" in the Journal of Applied Physics, Vol 41, page 2755 (1970).